1. Field of the Invention
The present invention relates to an array of semiconductor elements such as light-emitting diodes, and more particularly to the connections and layout of the wire-bonding electrode pads of the array.
2. Description of the Related Art
Linear arrays of light-emitting diodes (LEDs) are used as light sources in, for example, electrophotographic printers. FIG. 18 shows the cross-sectional structure of one such array; FIG. 19 shows a plan view of the array. These drawings are taken from page 60 of LED Purinta no Sekkei (Design of LED printers), published by Torikeppusu. In the illustrated LED array 100, a p-type impurity such as zinc has been selectively diffused into an n-type gallium-arsenide-phosphide (GaAsP) semiconductor layer 101 through windows in a dielectric film 102, aluminum p-electrodes 103 have been formed on the dielectric film 102, and a common gold-germanium-nickel (Auxe2x80x94Gexe2x80x94Ni) n-electrode 104 has been formed on the underside of an n-type gallium-arsenide (GaAs) substrate 105 to create an array of LEDs 106. Each p-electrode 103 couples an LED 106 to a p-electrode pad 107 having sufficient area for wire bonding. The LED 106 emits light when a forward voltage is applied between this p-electrode pad 107 and the common n-electrode 104. The LEDs can thus be individually driven to create a pattern of dots on a photosensitive drum in a printer.
One problem faced by this type of LED array is that if the array density is increased to improve the printing resolution, the p-electrode pads 107 must be must be made smaller and packed more closely together, or staggered in double rows as illustrated in FIG. 19. As a result, wire bonding becomes more difficult, the yield of the manufacturing process is lowered, and the cost per array rises.
Other problems arise because current must be driven through the substrate 105, even though no light is emitted from the substrate. Attempting to solve these problems by replacing the common n-electrode 107 on the underside of the array with one or more n-electrode pads on the upper surface of the array would only aggravate the wire-bonding problem.
An object of the present invention is to provide an array of semiconductor elements that can be efficiently driven and easily wire-bonded.
The invention provides a semiconductor device having a substantially linear array of semiconductor blocks of a first conductive type. Each semiconductor block includes a diffusion region of a second conductive type, and has a first electrode, separated from the diffusion region, making electrical contact with the semiconductor material of the first conductive type. The provision of a first electrode in each semiconductor block reduces differences in electrical characteristics between different semiconductor blocks.
The semiconductor device also has a plurality of first electrode pads, each having a pair of electrode leads. The leads connect the electrode pad to two semiconductor blocks: one lead makes electrical contact with the diffusion region in one of the two semiconductor blocks; the other electrode lead makes electrical contact with the first electrode in the other one of the two semiconductor blocks. When placed at one potential, the electrode pad activates the one of the semiconductor blocks; when placed at another potential, the same electrode pad activates the other one of the semiconductor blocks. Each semiconductor block is electrically coupled to just one of the first electrode pads. Time-division driving of the semiconductor blocks is simplified because each first electrode pad drives two semiconductor blocks.
The remaining diffusion regions and first electrodes in the semiconductor blocks may be coupled in a similar fashion to a plurality of second electrode pads. In an alternative scheme, the semiconductor device has a single second electrode pad coupled to the remaining diffusion regions, and a third electrode pad coupled to the remaining first electrodes. In another alternative scheme, the semiconductor device has a single second electrode pad coupled to all of the remaining diffusion regions and first electrodes. The alternative schemes enable the semiconductor device to be driven efficiently by a comparatively small number of electrode pads.
The semiconductor blocks are preferably oriented so that the first electrode is separated from the diffusion region in a direction orthogonal to a longitudinal direction of the array, as this orientation enables the array pitch to be reduced.
The semiconductor blocks are preferably isolated from one another by trenches, or by an isolation diffusion region of the second conductive type. The resulting electrical isolation has the desirable effect of confining driving current to the semiconductor blocks. If an isolation diffusion region is used, it can be formed at the same time as the diffusion regions in the semiconductor blocks, simplifying the fabrication process.
The width of the first electrode pads is preferably less than twice the array pitch, so that the first electrode pads can be arranged in a single row to simplify wire bonding.
The semiconductor blocks may emit light when activated.